Long before a wave of water rushed down the Bhote Koshi river and its tributaries, researchers were working to calculate just how many people are living downstream and exposed to potentially catastrophic flood risk in the areas that drain from Nepal’s high mountain basins.
The number they came up with — about 294,000 and growing — shows just how many people are living in areas of extreme risk in a country where climate change is destabilizing the rock and ice holding Himalayan slopes together and where meltwater is pooling in an ever-growing catalog of glacial lakes.
“For Nepal, it shows that there’s a lot of development going on very close to the river channel in these potentially hazardous zones,” said Scott Watson, a senior research fellow at the University of Leeds in England who studies glacial lakes. “That’s driven by tourism and the trade routes. The recent disaster was on the main trade route between Nepal and the Tibetan Plateau.”
The August disaster — when a massive rock and ice avalanche plunged off an almost 24,000-foot peak called Langtang Lirung and caused a debris flow that traveled about 120 miles — killed at least 1,455 people, and at least 5,285 people are still missing, according to Nepali authorities. Nearly 14,000 people were rescued.
If nothing is done, researchers say the number of people exposed will only grow.
“Hopefully, for Nepal, it’s kind of a bit of a wake-up call, this unfortunate recent disaster, in that there needs to be obviously a lot more focus on thinking about early warning systems, but also land use zoning,” Watson said. “We show that there’s a lot of buildings, there’s a lot of people very close to the river channel that are exposed to these hazards.”
About 108,000 of those people are downstream of glacial lakes, the researchers found. Many of those glacial lakes are pools of water dammed by ice that could fail and release a flush of water all at once.
The researchers started putting together this analysis in fall 2025, but it spent months undergoing academic peer review. They published their findings for the first time in the journal Scientific Reports on Thursday. The research is best viewed as a rough draft of where flood risk exists near Nepal’s high peaks — more thorough analysis is still needed.
The researchers performed their analysis by building a map of all rivers that originate in the transboundary areas of Nepal along the border with Tibet and India. They isolated areas within 50 kilometers, or 31 miles, downstream of glacial lakes or the headwaters of a river basin. Then, they mapped what would be exposed to floodwaters in a flood with 50 meters, or 164 vertical feet, of rising water downstream.
The researchers found about 108,000 people downstream of glacial lakes with ice that could fail and send a pulse of floodwater racing downstream.
Floods that rise 164 vertical feet and travel 31 miles might sound like an extraordinary occurrence. However, the August flooding in the Bhote Koshi river and its tributaries traveled about 120 miles downstream and raised water levels by roughly 330 feet in some communities, said Dipesh Chapagain, a senior scientist at the United Nations University Institute for Environment and Human Security.
“The risk could even be higher” than the paper suggests, said Chapagain, a Nepali scientist who studies climate change and flooding. “This event set a new level, a new threshold.”
Unlike the U.S., detailed flood hazard mapping is not available for much of Nepal. Watson said previous efforts to understand what portion of Nepal’s population is exposed to dangerous flooding that originates in the high mountains simply drew 1-kilometer (0.62-mile) buffers around alpine rivers.
Watson said the new research will inform where flood hazard mapping should be prioritized.
“We’re collecting very high-resolution elevation models to run some of the detailed flood hazard maps so we can focus on the most exposed hot spots,” Watson said. “To get into that level of detail, we kind of had to have this screening tool to say, you know, where are the hot spots?”
After more than a month of searching for missing people in the Bhote Koshi flooding, Nepal called off its search. The focus now, Chapagain said, is to build temporary settlements for thousands of flood victims. There is not a concrete plan yet for how to approach permanent resettlement, but Chapagain said new construction must be prioritized in safe areas.
“Future planning should consider all these risks when planning for new infrastructure projects, new settlements and also for the flood recovery projects,” Chapagain said, adding that flood early-warning systems should be prioritized in areas this study identified as high-risk.
Watson said the study shows that too much construction and development in Nepal has not been regulated. Hydropower investment and tourism are booming in Nepal, which has brought paved roads into high mountain catchments. The study found that the amount of area built within 50 vertical meters of riverbanks increased by approximately 46% from 2016 to 2023.
“Once those communities start to form, the amenities soon follow, your schools, your healthcare — you can quite quickly end up with a growing community in an area that’s actually very close to the river channel,” Watson said.
Meanwhile, he said thousands of glacial lakes are forming upstream as glaciers disappear and permafrost thaws due to climate change — and many could send a pulse of floodwater downstream with little notice or warning.
